A heating resistor type air flow meter of prior art related to the present invention will be described with FIG. 2. This prior art example is disclosed in Japanese Patent Laid-Open No. H9 (1997)-203650.
A heating resistor 1 is formed by a temperature-sensitive resistor having a temperature-dependent resistance characteristic and placed in an passage of airflow to be measured. Actually, the heating resistor 1 is provided in a Wheatstone bridge circuit which is not shown, together with a flow rate signal producing element (resistor 2) and a temperature-sensitive resistor for air-temperature compensation, which is not shown. A heating current flowing through the heating resistor 1 is controlled by a heating current control circuit 10 to maintain the heating resistor 1 at a predetermined heating temperature (resistance value) with regard to the air-temperature.
Since this heating current varies depending on air flow rate, the heating current is converted into a voltage as an air flow rate signal V2.
The heating current flows through the heating resistor 1, even when the flow rate of the air is zero. Therefore, the sensed flow rate signal V2 is subjected to zero adjustment as well as amplification using an output control circuit 20, and a flow rate characteristic signal Vout is output. A reference voltage V0 required for zero adjustment is supplied by an internal reference voltage generator circuit 30.
In the resistor elements and the internal reference voltage generator circuit which constitute the air flow meter, characteristics of resistance or the like thereof vary with changes in circuit temperature, resulting in variation in the flow rate characteristic signal Vout. Therefore, it is needed to compensate its temperature characteristic (output variation).
Such a characteristic compensation is performed using a plurality of voltage dividing resistors 31, 32, 33, 34, and one of which is a resistor for adjustment. Variations (gradient) in outputs of the flow meter at known two-point temperatures are observed beforehand, and the resistance value of the resistor for adjustment is adjusted to produce an offset signal to offset the variations in the output. Moreover, Variations (non-linear line) in outputs of the flow meter at known three-point temperatures is observed beforehand, and the resistance value of a resistor 35 is adjusted to output a signal for compensating for the linearity. The adjustments as above are easily implemented by a laser trimming method.
The internal reference voltage generator circuit 30 is configured by a bandgap reference voltage generator circuit utilizing the bandgap of silicon. This voltage generator circuit 30, the heating current control circuit 10, and the output control circuit 20 are integrally formed in a semiconductor integrated circuit (IC) 3 except for some of the resistors.
In the above circuit configuration, if the temperature characteristic compensation circuit is constructed by using transistors integrated into the IC 3, the number of components of the flow meter can be reduced and the circuit board can be made smaller.
The adjusting resistor for temperature characteristic compensation is external to the IC 3 because of its adjustment function. In the bandgap reference voltage generator circuit, it is unsuitable that only the adjusting resistor is external to the IC and other resistors are integrated into the monolithic IC for the following reason.
The resistors integrated into the monolithic IC are affected by variance in diffusion, which results in variation in the temperature characteristic of the reference voltage. Consequently, it is difficult to adjust the characteristic of the flow meter. Therefore, if the adjusting resistor in the bandgap reference voltage generator circuit is external to the IC, all resistors in the voltage generator circuit need to be external to the IC to ensure the precision of the flow meter.
For this reason, in the prior art embodiment of FIG. 2, the adjusting resistors 31, 32, 33, 34 for adjusting the flow meter characteristic at two-point temperatures are external to the IC. Besides, the adjusting resistor 35 for linearity compensation for the bandgap reference voltage generator circuit (with a temperature coefficient different from that of the above adjusting resistors) is also incorporated into an external circuit with the adjusting resistors. In consequence, it is necessary to equip the IC with three or more terminals. Increase in the number of connection terminals of the IC becomes a bottleneck in reducing the dimensions of the air flow meter and reducing its cost.
[Patent document 1] Japanese Patent Laid-Open No. H9 (1997)-203650